Levulinic acid is a starting molecule for the synthesis of chemicals useful in various applications, including fuel additives, plasticizers, and solvents. For example, levulinic acid can be used to synthesize methyl tetrahydrofuran (MTHF), delta-amino levulinic acid (used as herbicides and pesticides), diphenolic acid (used to synthesize polycarbonates), succinic acid (used to make polyesters), and gamma valerolactone (5-methylbutyrolactone) (used for production of adipic acid (1,6-hexanedioic acid)).
Levulinic acid can be produced by acid hydrolysis of C6 sugars, which is readily present in lignocellulosic biomass. Lignocellulosic biomass also includes C5 sugars, typically bound to hemicellulose, which can be converted to furfural, and lignin.
Prior art methods for production of levulinic acid from woody biomass, such as those described in U.S. Pat. No. 5,608,105 and U.S. Pat. No. 4,897,497, disclose a two-step hydrolysis process for the production of levulinic acid from woody biomass. For example, U.S. Pat. No. 5,608,105 discloses a first hydrolysis step carried out for a short period at relatively high temperature followed by a second hydrolysis step carried out for a longer period at lower temperatures. These and other prior art methods for production of levulinic acid, however, suffer from significant disadvantages.
For example, prior art methods of hydrolysis of lignocellulosic biomass result in the formation of large amounts of tar and char, materials that are insoluble in water and become viscous and very dark to almost black when concentrated. Tar usually refers to a viscous liquid formed from destructive heating of organic material, for example by pyrolysis, and when carbohydrates are subjected to acid hydrolysis, particularly at high temperatures. Char usually refers to solid material, for example the remains of solid biomass that has been incompletely combusted, such as charcoal if wood is incompletely burned.
The formation of tar and char that is abundant in prior art processes is undesirable. For example, the dark color of tar and char makes the product unattractive (i.e., from the perspective of the user or customer) and the presence of tar and char may negatively affect the performance of the final product.
Another disadvantage of prior art processes is that the lignin present in the lignocellulosic biomass ends up as char. Lignin is a heterogeneous polymer of aliphatic and aromatic portions which is present in all kinds of wood. In paper manufacture, due to the harsh conditions, pulped lignin has undergone considerable chemical modification and is generally a dark brown to black brittle solid, and is generally only suitable as fuel. Lignin as a fuel source gives it a low added value. There are other higher value applications, such as the manufacture of paper, food additives (vanillin essence), fine chemistry, metallurgy, and emulsifiers; however, such applications are impossible when the lignin is in the form of char.
Still other disadvantages of prior art processes are that they are not suitable for high concentrations of biomass, do not produce products with high concentrations of levulinic acid, and require subsequent separation processes to separate the levulinic acid.
Thus, there remains a need for improved methods for producing levulinic acid and other desirable products from lignocellulosic biomass.